Restriction enzymes arose as bacterial defenses against viruses. They destroy DNA with specificity for characteristic sequences, which the bacterial genome can avoid. Restriction are crucial tools for the molecular biologist because not only do they cleave DNA at specific sequences, but the cleavage occurs with 'sticky ends' remaining on the cleaved DNA, a non-base paired sequence that can thus be combined with another fragment of DNA cleaved by the same enzyme.

Restriction enzymes recognize sites with twofold symmetry and are themselves symmetrical.

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